Header-Based DirectFromSellerSignals Feature Detection

[JacobGo]

To run an experiment using the header-based directFromSellerSignals spec proposed in #695, we must detect whether this API is supported to avoid moving auction signals into the header server-side with no corresponding client-side mechanism to retrieve them in the auction worklet.

Ideally the client should have a JS visible mechanism that it may invoke to indicate to the server that it can support parsing the Ad-Auction-Signals header and making it available within the auction worklet by providing directFromSellerSignalsHeaderAdSlot.

[JensenPaul]

It'd be nice if we had a dedicated mechanism to detect APIs like this, but in the mean time I think this should work:

let dfss = false;
navigator.runAdAuction({get directFromSellerSignalsHeaderAdSlot(){dfss = true;}}).catch((e)=>{});

Seems to run pretty quickly, close to 10μs in a loop. I played with a few different ways, but this seemed fastest for a few reasons:

1. By having an invalid config (missing seller and decisionLogicURL) it avoids doing an IPC which makes it >10x faster
2. Avoids await by using Promise.catch
3. Avoids try {} catch {} by using Promise.catch

[dmdabbs]

Thanks @JensenPaul.

If I grok this, it is relying on an attribute fetch within the auction mechanics (to determine whether the config expects DFSSAS inputs?) that (currently) happens prior to basic auction config attribute validation.

I tried to locate this via the DFSS issue (https://bugs.chromium.org/p/chromium/issues/detail?id=1462720) CRs but could not find them. Would you or @caraitto kindly provide a pointer? Nice source browser & tools, BTW!

If I follow the comments in that issue, DFSSAS impl is landing with M118, which is now in Beta channel. Should your snippet work on that version, perhaps with some extra flag enabled?

[caraitto]

@dmdabbs

I tried to locate this via the DFSS issue (https://bugs.chromium.org/p/chromium/issues/detail?id=1462720) CRs but could not find them. Would you or @caraitto kindly provide a pointer? Nice source browser & tools, BTW!

This happens in the generated code produced from WebIDL [0]. The WebIDL definition is here [1], and the code generated from the WebIDL that actually reads the property (and therefore executes the get JavaScript code) is here [2] (it's the Chrome OS version, but it should be basically the same on other platforms -- the generated code isn't checked into git but instead it's produced by the build, but Chromium Code Search still indexes it).

[0] https://developer.mozilla.org/en-US/docs/Glossary/WebIDL

[1] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/modules/ad_auction/auction_ad_config.idl;l=42;drc=05bbf9bc1b9dff6e19ae15092fe095e33208ea13

[2] https://source.chromium.org/chromium/chromium/src/+/main:out/chromeos-Debug/gen/third_party/blink/renderer/bindings/modules/v8/v8_auction_ad_config.cc;l=1042;drc=31fb07c05718d671d96c227855bfe97af9e3fb20

If I follow the comments in that issue, DFSSAS impl is landing with M118, which is now in Beta channel. Should your snippet work on that version, perhaps with some extra flag enabled?

We cherry-picked [0] into Chrome 117.0.5938.35; however, currently the feature FledgeDirectFromSellerSignalsHeaderAdSlot is disabled by default. We are working to get approval on our intent to ship (I2S) [1] so that we can turn the feature flag on.

After we turn the flag on, Chrome browser instances will turn on the feature after a browser restart. Typically there is no user prompting UX to restart like there is when an update is ready to be installed. Chrome channels like Canary, Dev, Beta, and Stable can also be targeted separately at different percentages to provide a smooth, stable rollout.

The feature can be enabled locally using this command line switch [2]:

--enable-features=FledgeDirectFromSellerSignalsHeaderAdSlot (it's a comma-separated list of features)

[0] https://crrev.com/c/4803170

[1] https://groups.google.com/a/chromium.org/g/blink-dev/c/JpWOdoPi5Wo

[2] https://www.chromium.org/developers/how-tos/run-chromium-with-flags/ (also applies to official Chrome builds)

[caraitto]

By the way, there's an open WebIDL feature request to better support feature detection for IDL dictionary members: whatwg/webidl#107.

[dmdabbs]

Thank you for the quick and detailed responses, Caleb (@caraitto)!

Good that Chrome's FillMembersFromV8Object processes attributes in alpha order.

[caraitto]

Yeah, we're lucky directFromSellerSignalsHeaderAdSlot comes before seller alphabetically -- otherwise, the seller field would have to be passed to the detector, and we'd need some other way to get the C++ to throw to avoid IPC -- we'll definitely want a better long-term solution :)